Intermittent Heat

[onecrazyblackredenck]

I have been battling this for two years now so its time to get yalls help. Under load my truck has great heat, but as soon as I get off the pedal, the heat turns cold especially when I stop at a red light. I have replaced the thermostat, flushed the coolant system and replaced the degas cap. Is this a vacuum issue or sensor or.............

[kinnisoj]

Alex, "Ill tale air in the heating system for $100 plz"

[southend]

One of the signs of head gasket failure, iirc.

Sent from my phone that somebody didn't help me get.

[Evil Eye]

Originally Posted by southend

One of the signs of head gasket failure, iirc.

Sent from my phone that somebody didn't help me get.

^^^^This is something I have also heard.

[mhatlen]

Yea the combustion gasses enter cooling system and since the heater core is the highest portion of the cooling system the gasses wind up in it, high flows (rpm,s) will push more coolant through the heater core.

You can take a look at your heater control valve is it vacuum operated or mechanical ?
Make sure it is completely open with heater on. ( it's above pass valve cover near firewall and has 3/4 hose through it lol ) just make sure it's moving completely open. Suppose it's possible to do what your describing if it's only partially open.
Obviously check that first, hope it's this simple.....

[dannyboy950]

The heater core is designed for a hot liquid to pass thru its coils relatively slowly give it a chance to transfer heat to the air. Hot gases even tho hotter pass thru the coils too quickly to properly transfer the heat. The gases also generate more pressure and keep the coolant level down from being able to flow thru the core. A simplified explanation taken from my grandsons science book. LOL

[onecrazyblackredenck]

Dang , I was hoping it wouldn't be head gaskets. Is it a must to pull the cab or motor to replace these gaskets ? If my headgaskets are bad shouldn't I have milky oil ? What would be some other symptoms ?

[southend]

Originally Posted by onecrazyblackredenck

Dang , I was hoping it wouldn't be head gaskets. Is it a must to pull the cab or motor to replace these gaskets ? If my headgaskets are bad shouldn't I have milky oil ? What would be some other symptoms ?

No you can do it in frame, read the stickies for a writeup Tea kettle whistle, fluid level in the degass bottle pushing down, leaking hoses from overpressurization, hydrolocking, loss of coolant (sometimes), there are others i'm forgetting. Look up 6.0 head gasket testing. Oil in the coolant would be ruptured oil cooler. You need a monitor bud. Read the stickies.

Sent from my phone that somebody didn't help me get.

[Term3]

Rule out a leaking EGR cooler first.

[onecrazyblackredenck]

Originally Posted by southend

No you can do it in frame, read the stickies for a writeup Tea kettle whistle, fluid level in the degass bottle pushing down, leaking hoses from overpressurization, hydrolocking, loss of coolant (sometimes), there are others i'm forgetting. Look up 6.0 head gasket testing. Oil in the coolant would be ruptured oil cooler. You need a monitor bud. Read the stickies.

Sent from my phone that somebody didn't help me get.

Thanks, I dont have any of those symptoms, thats why I was surprised to see that head gaskets could be the culprit. I will do some more reading to try and diagnose better.